MESFETs (metal semiconductor field effect transistors) include a conducting channel positioned between source and drain contact regions. The carrier flow from the source to drain is controlled by a Schottky metal gate. The channel is controlled by varying the depletion layer width underneath the metal contact which modulates the thickness of the conducting channel and thereby the current. MESFETs require a relatively low on resistance (Ron) to achieve good performance. In addition, the blocking capability of GaN MESFETs on Si substrates is limited by the thickness of the GaN layer because accumulation of charge carriers occurs in Si on the boundary surface between GaN and the substrate and thus cannot dissipate any voltage. However, the thickness of the GaN layer is limited because of the change in stress from GaN to Si.
Current power transistors based on GaN are constructed mostly as HEMTs (high electron mobility transistors) which are also known as heterostructure FETs (HFETs) or modulation-doped FETs (MODFETs). An HEMT is a field effect transistor with a junction between two materials having different band gaps such as GaN and AlGaN which forms the channel instead of a doped region such as in a MOSFET (metal oxide semiconductor field effect transistor). HEMTs provide a 2-dimensional electron gas (2DEG) which is formed on the boundary between e.g. an AlGaN barrier layer and a GaN buffer layer. Without further measures, such a construction leads to a self-conducting i.e. normally on transistor. That is, the HEMT conducts in the absence of a positive gate voltage.
The conductive band can be raised in such a way e.g. with a p-type AlGaN or p-type GaN layer under the gate electrode which shifts the resulting applied voltage in comparison to a device without such a p-type layer by about 3V (equivalent to the band distance) in the positive direction. An HEMT with such a construction is a normally off transistor in that a positive gate voltage is needed to switch on the transistor. Alternatively, a recessed gate structure can be used to ensure the HEMT functions as a normally off transistor.
In each case, inexpensive silicon substrates are typically used to fabricate an HEMT. However, for high voltages of more than 100 V, the entire voltage must be absorbed in the thickness of the GaN layer because the substrate cannot dissipate any voltage as described above. HEMTs also typically have a planar gate structure, limiting the channel width.